1. Field of the Invention
The present invention relates, in general, to a catalyst for removing dioxin. More specifically, the present invention relates to a catalyst having high removal efficiency of dioxin, which is prepared by recycling an alumina-based spent catalyst discharged from a hydro-desulfurization process of an oil refinery.
2. Description of the Prior Art
Exhaust gases generated, for example, from incineration facilities for disposing of industrial wastes and municipal wastes, steel-making plants and metal refining industries, contain trace amounts of toxic organic chloro-compounds, such as dioxins, PCB and chlorophenol, in addition to harmful substances, such as sulfur oxides (SOx), nitrogen oxides (NOx), and chlorine.
Dioxins, known to be poisonous substances, which cause various cancers and birth defects, are generated when chemical wastes containing dioxin precursors, such as municipal wastes and medical wastes, are incinerated in incinerators such as refuse furnaces and recycling facilities. The substances referred to as dioxins are grouped into dioxin-based compounds (PCDD: polychlorinated dibenzo-p-dioxin), in which two benzene rings containing two replaced chlorine atoms are connected through two oxygen atoms, and furan-based compounds (PCDF: polychlorinated dibenzofuran) which is connected through one oxygen atom. Dioxins include their isomers which include as many as 75 types for dioxin-based compounds and 135 types for furan-based compounds depending on the position and the number of replaced chlorine atoms. In other words, a total of 210 dioxin compounds are present. Among them, 2,3,7,8-TCDD is known as the most toxic substance versus the human body and the environment, remaining in the human body and the environment for a long time. Further, since dioxins are highly stable water-insoluble substances and their toxicity is substantially permanent, they are considered to be the most significant chemical substance that causes environmental pollution. It is reported that formation of dioxins depends on temperature, chlorine (HCl, Cl2) concentrations, oxygen and carbon monoxide concentrations on incineration and thermal treatment. Dioxins are formed in a maximum amounts at 250-450xc2x0 C. in de novo synthesis, but decomposed at 600xc2x0 C. or higher.
Dioxins generated from incinerators can be treated in two manners: a pre-treatment technique and a post-treatment technique. In case of the pre-treatment technique, dioxin discharge may be reduced through previous separation of wastes, structure alteration of combustor and optimization of combustion conditions but its generation cannot be totally prevented. In view of foregoing, the synthesized dioxins may be effectively removed through the post-treatment technique in combination with the pre-treatment technique.
In case of the post-treatment technique, dioxins generated from treatment of exhaust gas and/or combustion are removed before being discharged to the atmosphere. These techniques classified into, for example a catalytic oxidation-decomposition, a thermal-incineration and decomposition, an adsorption-separation and so on. As for the catalytic oxidation-decomposition, exhaust gas contacts with catalyst such that dioxins are decomposed into substances such as CO2, H2O, HCl and so on. Recently, this technique has been under vigorous study. In a thermal-incineration and decomposition, exhaust gas containing dioxins is re-heated to high temperatures of 1000xc2x0 C. or higher, thereby decomposing dioxins, and such a technique as mainly controls combustion in the incinerators is disclosed in Korean Patent Laid-Open Nos. 98-019531 and 98-019532. However, said technique suffers from disadvantages of high cost for using energy and installing equipments, and also re-synthesis of dioxins during cooling after thermal-decomposition. So, this method is mainly used in treatment of ashes from the incinerators.
The adsorption-separation, which causes dioxin compounds to be adsorbed and removed by contacting exhaust gas with adsorbents may generate secondary pollutants due to recycling of adsorbents and treating of spent adsorbents. In this regard, Korean Patent Laid-Open No. 2000-41789 discloses a method for removing dioxins, in which dioxins are adsorbed and removed using polyethylene (PE) or polypropylene (PP) as adsorbents at constant temperature and then the used adsorbents are recycled. But, this method has problems as described above.
Meanwhile, through conventional techniques for removing dioxins in exhaust gas, smoke may be removed by a dust-collection of a fountain-form washer mounted to exhaust flues in the incinerators but dioxins adsorbed from discharge gas are not completely removed. Accordingly, such methods are rarely used.
Among post-treatment techniques, the catalyst oxidation-decomposition method in which transition metal compounds (TiO2, V2O5, WO3, Cr2O3, Co3O4, CuCr2O4) or noble metals (Pt, Pd) are used is most advantageous in terms of removal efficiency of dioxin. In this regard, U.S. Pat. No. 5,783,515 refers to a catalyst for removing dioxins prepared by impregnating zeolite with a first catalyst ingredient which is at least one element selected from the group consisting of Pt, Pd, Ir and oxides thereof, and a second catalyst ingredient which is at least one element selected from the group consisting of Au, Ag, Cu, Fe, Sb, Se, Te, Ta and oxides thereof.
However, the noble metals are expensive and also susceptible to poisoning by chlorine, thus transition metals being more widely used instead of them. In particular, in practical dioxin-decomposition processing, Pt/Al2O3-cordierite, V2O5xe2x80x94WO3/TiO2 monolith, TiO2 monolith and the like are used as catalysts. Therefore, useful as conventional catalyst for removing dioxins is a catalyst comprising a titanium oxide support impregnated with active metals, such as vanadium, tungsten and so on. However, a catalyst for removing dioxins, which has high decomposition activity versus dioxins, and excellent heat stability and poisoning resistance has not been developed yet. In addition, conventionally used titanium oxide, vanadium and tungsten are too expensive and thus economic burden may occur. Accordingly, a novel method for removing dioxin having more efficient and economic removal activity of dioxin is required.
Leading to the present invention, the intensive and thorough research on a spent catalyst discharged from a hydro-desulfurization process of an oil refinery, carried out by the present inventors aiming to avoid the problems encountered in the prior arts, resulted in the finding that a spent catalyst comprising an alumina support (preferably gamma alumina) with a large specific surface area impregnated with high contents (5 wt % or higher) of vanadium may be recycled to prepare a catalyst for removing dioxin. The spent catalyst is mixed with a tungsten-impregnated titania support to re-distribute the excess metal components contained therein into the tungsten-impregnated titania support. The thusly prepared catalyst may be used as a dioxin removal catalyst, which has high removal efficiency versus dioxin, thus incurring economic benefits.
Therefore, it is an object of the present invention to provide a catalyst having high removal efficiency of dioxin.
It is another object of the present invention to provide a method for preparing a dioxin removal catalyst, which may incur economic benefits by using a spent catalyst discharged from a hydro-desulfurization process of an oil refinery.
In accordance with one aspect of the present invention, there is provided a catalyst for removing dioxin, comprising 1-10 wt % of vanadium, 0.1-5 wt % of nickel, 0.1-5 wt % of molybdenum and 1-15 wt % of tungsten, on a mixture support consisting essentially of 10-50 wt % of alumina and 50-90 wt % of titania.
In accordance with second aspect of the present invention, there is a provided a preparation method of a dioxin removal catalyst, which comprises the following steps of:
a) pretreating a spent catalyst discharged from a hydro-desulfurization process of an oil refinery, which comprises 5-30 wt % of vanadium, 1-10 wt % of nickel, 1-10 wt % of molybdenum, 0.1-5 wt % of iron, 1-10 wt % of sulfur, 0.1-5 wt % of silicon and 0.1-5 wt % of phosphor on an alumina support by thermally treating said spent catalyst followed by washing with water;
b) providing a titania impregnated with 1 to 20 wt % of tungsten;
c) homogeneously mixing the pretreated spent catalyst with the tungsten-impregnated titania under the addition of water and acid;
d) dehydrating the mixture to remove excess moisture and active metal components therein;
e) drying the dehydrated mixture, followed by grinding the dried mixture; and
f) forming a catalyst body by extruding the grinded mixture or coating the grinded mixture to a structure, followed by drying and then calcining the dried structure.